400 HORTICULTURAL MANUAL. 



lands with clay subsoil are sub-irrigated for cranberry- 

 growing. Ditches surround the planted tracts, in which 

 water is introduced from running streams or from reser- 

 voirs. 



Close observation will disclose hundreds and even thou- 

 sands of these naturally sub-irrigated tracts in about every 

 State of the Union which have not as yet been utilized 

 for gardening or fruit-growing. 



404. Green-house Sub-irrigation. This has become far 

 more general than outdoor sub-irrigation. The trenches 

 are made water-tight by spreading cement over the slate 

 bottoms and sides. In the bottom drain-tile are laid in 

 which water circulates and slowly escapes at the joints and 

 through the pores of the tiles. Some lay the tiles two feet 

 apart and others much prefer laying them quite close to- 

 gether. Where the tiles do not fit well some cement is iised 

 to tighten the joints to prevent too much water escaping 

 at one point. The water is raised by capillary attraction 

 through the soil covering over the tiles. The benefits of 

 the system arise from the regularity of moisture supply, 

 and the surface does not harden or bake as in surface- 

 watering. 



Practical growers often start plants in shallow boxes and 

 secure irrigation from below by setting the boxes in a 

 shallow pan or vat with water in the bottom until moisture 

 appears on the surface. 



405. Surface Culture Needed. In all kinds of watering, 

 whether on the surface or from below, the surface must 

 be stirred soon after the wetting to prevent baking and to 

 conserve the moisture. If this is neglected with surface- 

 watering the outcome will prove doubtful as to benefit. 



Thorough culture and the exclusion of weeds tends to 

 hold the moisture in the soil. All systematic gardeners 

 cultivate soon after rains, but the need of it after surface 



